The present invention relates to a liquid crystal display apparatus, and more particularly to an active matrix liquid crystal display apparatus.
There are shown the liquid crystal display apparatus, for example, in Japanese Patent Publication No. 63-21907 (1988), UP, WO91/10936 and Japanese Patent Application Laid-Open No. 6-222397 (1994), in which a pair of comb electrodes are used to allow a direction of an electric field applied to a liquid crystal to be parallel with a surface of a substrate. However, in a display system wherein a direction of an electric field applied to the liquid crystal is controlled to a direction parallel with a surface of a substrate by using active elements (hereinafter referred to as a horizontal electric field method), there is not disclosed the characteristic of a light source required to decrease the power consumption of the whole liquid crystal display apparatus. Further, there is not disclosed the configuration of the liquid crystal display apparatus required to suppress the color shift due to the appliance of a voltage and improve the color defect.
In the horizontal electric field method, opaque electrodes are provided in a display pixel portion in order to apply the electric field substantially in parallel with the surface of the substrate. As compared with the prior art method wherein an electric field is applied in a direction substantially vertical to the surface of the substrate by using a transparent electrode, the aperture ratio may be deteriorated and the brightness under a bright state may be lessen. Accordingly, it is necessary to use a high-intensity light source in the horizontal electric field method.
Because the display mode effective for the liquid crystal display apparatus of the horizontal electric field is a double refraction mode, the transmittance T can be generally expressed by the following equation (1). ##EQU1## where, To designates a coefficient and is determined mainly by the transmittance of a polarizer, .theta. designates an angle between an effective optical axis in the liquid crystal layer and a transmittance axis for a polarized light, d thickness of the liquid crystal layer, .DELTA.n anisotropy of refractive index of the liquid crystal layer, and .lambda. wavelength of light. Because the transmittance of the liquid crystal display apparatus has essentially the maximum value in a certain wavelength, the liquid crystal display elements are colored. One solution of the above equation is a value which satisfies such the condition that the peak wavelength becomes equal to the maximum wavelength 555 nm for luminous efficiency under the retardation of 0 order, that is, (.pi.d ..DELTA.n/555)=.pi./2. At this time, the transmittance falls suddenly on a short wavelength side of the peak wavelength, and it decreases gradually on a long wavelength side. Therefore, the liquid crystal display elements are colored in yellow. As a result, it is required to use a light source with the color of a cold color family which is the complementary color to yellow. In other words, it is required to use the light source with a high color-temperature characteristic.
In general, a fluorescent lamp is used as a light source for a liquid crystal display apparatus. Because the luminous efficiency of the fluorescent lamp in a short wavelength region is less than that in a long wavelength region, the brightness may be lessen and large consumption power is required to obtain a high brightness. Since the normal voltage of the battery must be maintained for a long time, for example, in a note book type personal computer or a personal digital assistance, it is required to avoid the increase of the consumption power.
Now, a display operation of the liquid crystal display apparatus of a horizontal electric field method can be obtained in the double refraction mode, and the transmittance T can be generally expressed by the following equation (2). EQU T=T.sub.0 .multidot.sin.sup.2 2.theta..multidot.sin.sup.2 [(.pi..multidot.d.sub.eff .multidot..DELTA.n)/.lambda.] (2)
where, To designates a coefficient and is determined mainly by the transmittance of the polarizer used in the liquid crystal panel, .theta. designates an angle between an effective optical axis in the liquid crystal layer and a transmittance axis for a polarized light, d.sub.eff thickness of the liquid crystal layer, .DELTA. anisotropy of refractive index of the liquid crystal layer, and .lambda. wavelength of light. Further, the product of d.sub.eff and .DELTA. is called as retardation. Where, the thickness d.sub.eff of the liquid crystal layer is not the thickness of the whole liquid crystal layer, but the thickness of the liquid crystal layer in which the direction of alignment is changed when a voltage is applied.
In general, molecules of the liquid crystal in the vicinity of the boundary surface of a liquid crystal layer does not change the alignment direction owing to the effect of anchoring at the boundary surface even if a voltage is applied. Accordingly, when the thickness of the whole liquid crystal layer sandwiched between the substrates equals d.sub.eff, d.sub.eff &lt;d.sub.LC always is held between the thickness d.sub.LC and d.sub.eff. It is estimated that the difference between d.sub.LC and d.sub.eff equals about 20 nm to 40 nm.
As clearly seen from the above equation (2), the transmittance of the liquid crystal display panel takes the maximum value at a specific wavelength (peak wavelength). Therefore, the liquid crystal display element is easy to be colored, in other words, it is easy to be unnecessarily colored.
Generally, the liquid crystal panel is constructed so as that the peak wavelength may become equal to the maximum wavelength 555 nm for luminous efficiency, that is,(.pi.d ..DELTA.n/555)=.pi./2. At this time, the liquid crystal display element is colored in yellow, because the spectral transmittance falls suddenly on a short wavelength side of the peak wavelength, and it decreases gradually on a long wavelength side.
The extent of coloring extremely changes according to the appliance of a voltage to the liquid crystal. As the magnitude of the voltage value changes from the minimum voltage required to display to the medium tone display voltage and then the maximum voltage, the color tone is gradually changed. Therefore, the display state of colors is extremely deteriorated.
Because the difference between the thickness of the liquid crystal layers appears as the change in the peak wavelength in the birefringence mode, the local and abnormal thickness of the liquid crystal display layer causes display defects such as the variations of the intensity and/or color tone, which are different from those in its surrounding area.